Method of pickling and coating with vitreous enamel



Mali-ch 19,` 1963 R. A. HALvERsEN ETAL METHOD OF PICKLING AND COATING WITH VITREOUS ENAMEL Filed NOV. 20, 1961 cur/1w wirr/P ffm/sz( INF/VEA IN VENT-ORS. .F0 /Z /VaffycrJ-ezr United States Patent O 3,082,116 METHOD OF PICKLING AND COATING WIT VITREOUS ENAMEL Roy A. Halversen, Dearborn, and William S. Russell, Royal Oak, Mich., assignors, by mesne assignments, to Hooker Chemical Corporation, New York, N.Y., a corporation of New York Filed Nov. 20, 1961, Ser. No. 153,691 9 Claims. (Cl. 117-50) The present invention relates to phosphoric acid pickling processes and more particularly relates to an improvement in the method of spray phosphoric acid pickling which ena-bles the continuous sludge-free operation of such solutions on ferrous surfaces.

In a continuous processing of ferrous stock through aqueous acidic phosphoric acid solutions, as the metal surface is attacked, iron is dissolved in the form of ferrous ion. It is known that the ferrie ion acts as an accelerator to speed up the attack of a ferrous surface by a phosphoric acid solution and that the presence of a ferric ion is necessary in order to have a practical speed pickling operation. As the ferrie ion functions to promote the speed of metal attack, it is reduced from the ferric to the ferrous ion form, and thus adds to the quantity of ferrous ion in the solution. As the concentration of ferrous ion increases, the pickling effectiveness of the solution decreases, and it is therefore necessary to convert the ferrous to the ferrie for-m in order to have a continuous pickling which occurs at an economically satisfactory speed. It has long been recognized that the ferrous ion can be converted to the ferric ion by the use of oxidizing agents, such as nitrate, chlorate, hydrogen peroxide, and many others now well known to those skilled in the art.

The use of such oxidizing agents in pickling solutions is difficult to control and is at best considered generally unsatisfactory for large-scale commercial processing. Such procedures produce a quantity of ferrie ion in the solution which varies greatly and quite often exceeds the solubility limit of the ferrie ion in `the particular solution and thereby causes the formation of ferric phosphate sludge which is bulky, tends to cake on coils and nozzles, land to disrupt the effective etching `action of the solution. The variation in ferrie ion content also causes an uneven rate of pickling and less uniformity in the resulting pickled surface. Therefore, these excess concentrations of the ferrie ion also are to be avoided. On the other hand, the most practical operating concentration of the ferric ion is that which produces the highest rate of -rnetal attack and this concentration is one which closely approaches the maximum solubility limit of the ferric ion.

It istherefore, the primary object of this invention to provide a phosphoric acid etching process which can be continuously operated to pickle ferrous surfaces without the formation of undesirable ferric phosphate sludge.

Another object of this invention is to provide a phosphoric acid pickling process which enables the maintenance of the highest rate of pickling while simultaneously providing a uniform and consistent metal loss on `the surfaces processed therethrough from piece to piece.

A still further object of this invention is to provide an improved method for operating phosphoric acid pickling solutions which is more economical than heretofore known methods, in the respect that it does not build up ferrous dihydrogen phosphate as the process is continued.

Another object of this invention is to provide la pickling process which is especially suitable for preparing ferrous surfaces to receive a vitreous enamel coating with or without an intervening layer or coating between the pickled surface andthe vitreous enamel layer.

A still further object of this invention is to provide an improved vitreous enameling process which includes as lCe ' one of its steps the phosphoric acid pickling process of this invention.

The present invention is based on the discovery that the above and related objects are completely realized by maintaining the concentration of the ferric ion in the aqueous acidic phosphoric acid pickling solution at the optimum concentration by the positive step of controlling the proportion of ferrous ion which is present in the solution at any particular instant.

In [accordance with this invention the continuous control of the ferrous ion concentration in the pickling solution is effected by maintaining concurrent control of the rate of and quantity of conversion of the ferrous ion to the ferrie ion. The process of this invention employs the step of spraying the pickling solution on the work to be pickled, and this spraying converts some of the ferrous ion to the ferrie ion. The rate of conversion from the ferrous state to the ferrie statekhas been found to be primarily dependent upon the spraying conditions, that is to say, the conditions which control the degree of aeration of the pickling solution, namely the size of the sprayed particles and the oxygen availability for those particles. As the size of the particles decreases and as the quantity of oxygen and the time of contact of the particles with that oxygen increases, the rate of conversion from the ferrous to ferrie state also increases. The quantity of conversion is primarily dependent on the concentration of the ferrous ion in the solution being sprayed. For example, for any given spray condition, the quantity of ferrie ion formed from ferrous ion `during spraying increases as the ferrous ion concentration increases. -It will be apparent to those skilled in the art that the factors which control the rate and quantity of conversion are closely interrelated and are at least partially interdependent. This invention contemplates an overall control comprising the concurrent vmodification and `control of both the spraying conditions and the quantity of ferrous ion in the pickling solution on a continuous basis, as well as the separate modification of only the spraying conditions or control of the quantity of ferrous ion in the solution. The actual steps taken to effect the desired control will normally be dependent on the particular installation and will be partially a matter of preference. In many commercial installations, once the installation is completed, the spraying conditions remain fixed, but it will be appreciated that pickle solution ow rates, pressuresused on the solution, number of spray nozzles operative, etc. can readily be varied, if desired.

It has been found that satisfactory direct control of the ferrous ion concentration can be obtained Iby cycling a portion of the pickling solution through an ion exchange resin bed which is capable of removing the ferrous ion therefrom and returning the substantially ferrous-ion free solution to the operating pickling tank.

As broadly suggested above, a phosphoric acid pickling solution when pickling ferrous stock is constantly undergoing change in the ionic status and quantity of iron in the solution. The most effective aqueous phosphoric acid pickling solution for batch-type operation is one which contains a quantity of ferric ion barely below the saturation value for Vferric ion under the conditions of pickling being employed and la sufficiently low ferrous ion concentration to avoid reduction in the rate of metal attack. For continuous opera-tion, however, it has been found in accordance with this invention, that the pickling solution should contain a maximum of about of the saturation value of ferrie ion in the solution under the conditions being employed, and that the ferrous ion must be present in a minimum quantity corresponding to that which is necessary to keep the ferric ion concentration in the range about 13% to about 85% of the ferric ion saturation value underthe prevailing spraying conditions,

and preferably in the range of 50% to 80% of the ferric ion saturation value.

In order to better understand the inter-relationship of the ferrous and ferric ion concentrations, let it be assumed that a freshly prepared aqueous phosphoric acid solution is used to pickle ferrous stock, c g. low carbon steel, under constant spraying conditions and a constant rate of metal throughput, with the metal to be pickled being substantially similar in original condition. Let it be further assumed that the spraying is effected in an open-to-air spray booth and the solution, as prepared, contains about 15% H3PO4 and a small quantity of ferric ion, e.g. about 0.02% w./v. When the pickling operation is started, the rate of metal attack is relatively slow and the quantity of ferrous ion dissolved is low. As metal attack occurs, however, the lferrous ion in the solution tends to increase yand a portion of this ferrous ion is oxidized by the spraying to the ferric state. As the ferric ion concentration begins to increase, the rate of metal attack increases and more ferrous ion is dissolved per unit of time. As the ferrous ion dissolved in the pickling solution continues to increase, the ferric ion likewise increases and, in the absence of other inuences, the ferric ion will vultimately attain a concentration in excess of its saturation value and insoluble ferric phosphate precipitates in the solution. As work is processed through the spray booth, a portion of the piokling solution is carried away on the surface o-f the pickled stock and a certain quantity of both the ferrous and ferric ion are thus removed. While such drag-out does affect the quantities of iron in the pickling solution in ionic form, it affects the ferrous and ferric ions in a comparable fashion and thus does not alter the fact that there is, for any given particular installation, a critical ratio of the concentration of ferrous ion to ferric ion which, when exceeded, `under the above recited fixed conditions, will ultimately cause undesirable sludge formation as pickling is continued under the same conditions. The rate of build-up of 'ferric ion toward saturation rapidly increases when that critical ratio is exceeded and it is for this reason that the maintenance of optimum conditions during steady state continuous operation is extremely diicult.

In any continuous commercial operation the rate of throughput of the metal surface to be pickled varies as the result of numerous conditions outside the pickling operation which are impossible to control, and in controlling the pickling solution it is necessary to acknowledge and accept this fact. Variations -in the rate of throughput, where other conditions are constant, cause a variation in the relative quantities of ferrous and ferric ions in the pickling solution. Let it be assumed that a six liter phosphoric acid pickling solution is in steady state operating condition and contains 0.5% ferric ion, 0.3% ferrous ion and `is pickling low carbon steel at the rate of about 1 square foot per minute when an interruption in the supply of steel occurs and the throughput becomes zero. When the throughput stops, no additional quantity of ferrous ion will `be added to the solution from metal pickling and none of the ferric ion will be consumed in promoting metal attack. However, as the spraying continues, the ferrous ion in the solution is oxidized to the ferric state, the proportion of ferric ion relative to ferrous ion increases and with continued zero throughput the ferric ion solubility can be exceeded with consequent sludging. Under these conditions, the rate of conversion of ferrous to ferric ion can be decreased by direct reduction in the ferrous ion content or by reducing the rate of spraying or quantity of spraying or a combination of them.

At the other extreme, assume the same steady state operation and a doubled rate of throughput which becomes steady at the doubled rate. Due to the greater quantity of metal attack, more ferrous ion is dissolved. Some of the ferric ion is reduced to ferrous ion in promoting `metal attack and the over-all result is that the ferrous ion tends to rise in concentration. As the ferrous ion concentration rises, the rate of conversion to ferric ion increases and as processing continues, both the ferrous Iand ferric ion concentrations tend to increase. In this situation, new balanced steady state operation requires continuous removal lof a portion of the ferrous ion to avoid ultimate sludging.

In initiating a spray phosphoric acid pickling process with a new phosphoric acid solution the build-up of a ferric ion concentration to an efficient operating level by permitting the spray to form ferric ion from ferrous ion dissolved by metal attack is slow and time consuming. The time to reach an eilicient steady-state condition can be Igreatly reduced by dissolving iron in the solution as ferrous ion. It is feasible to add ferric ion directly to the new solution, but the degree of control which must be exercised to get the needed balance between the ferrous and ferric ion is thus increased and the preferred procedure is to start with a solution having a relatively high ferrous ion content. It is possible to pickle with an aqueous phosphoric acid solution containing as little as 0.02% w./v. ferric ion, but for purposes of a continuous operation it is preferred that the ferric ion concentration 4be at least 20% of the saturation value forferric ion in the solution involved. The ratio of ferrous to ferric ion, under conditions cornparable to initiation conditions, may be as high as about 11 ferrous ion to 1 ferric ion and steady state conditions still easily reached. Under conditions of operation in which the ferric ion is approaching, or exceeds, about of the solubility limit it may become necessary to reduce the ratio of ferrous to ferric ion to as low as about 1:7 in order to prevent the ferric ion from exceeding the solubility limit. Between these extremes, steady state operation can be maintained for the majority of installations by controlling the ratio of ferrous to ferric ion in the solution within the range of about 1:4 to about 4:1.

The saturation concentration of the ferric ion in an aqueous phosphoric acid solution increases as the phosphoric acid concentration increases. In continuous operation, therefore, it is necessary to exercise the control steps to alter the ferrous to ferric ion ratio at times which will prevent ferric ion saturation values from being attained. As a general guide, Table I sets forth illustrative ferric ion solubility val-ues. All values are expressed as percent weight/volume.

TABLE I Ferrie Ion Ferrie Ion Concentration Concentration After Opera- HgPO., Concentration At F. tion At 180 F.

Under Operatand Cooling ing Conditions At Least 10 Hrs. to 77 F.

From the standpoint of ultimate surface condition resulting from picklin'g, it is feasible to pickle the surface with a very dilute aqueous phosphoric acid solution, for example, a solution containing about 1% H3PO4. When Such dilute solutions are used, however, the time required to obtain the desired degree of pickling is longer than is required when somewhat more concentrated solutions are used, and in commercial production it is of course desirable to use the acidity which produces the desired degree of pickling in the least period of time. It is feasible to use concentrated solutions containing up to about 45% H3PO4. For most purposes a phosphoric aicid solution containing between about and about 30% i-s satisfactory.

The ion exchange resins which have been found to be suitable for the purposes of this invention are cation exchange resins, generally designated as `strongly acidic cation exchange resins such, for example, as the styrenedivinylbenzene resins which have been sulfonated with sulfonic acid in the nuclear positions. Such resins are commercially available from a variety of sources, and one such resin which has been found to be completely satisfactory for the purposes of this invention is Dowex 50, which is available from the Dow Chemical Company. A substantially similar resin is available from the Rohm and Haas Corporation under the designation of Amberlite Ill-120, and from the National Aluminate Company under the designations Nalcite HGR and Nalcite HDR. These resins are available in a sodium or hydrogen form and can be used in either form for the purposes of this invention although the hydrogen form is preferred. These resins having as little as 1% of divinylbenzene and as high as 16% divinylbenzene as a cross-linking agent can be 'used7 but it is preferred to employ resins containing about 4% to about 8% divinylbenzene as the cross-linking agent with styrene. While these resins can be obtained in extremely small mesh sizes, the most practical ferrous ion removal is obtained from the use of resins having mesh sizes between about 20 and 200 so that resins in these forms are preferred. The resin can be regenerated by conventional processing in a manner Well known to those skilled in the art, such as treating the resin with a concentrated mineral acid such as sulfuric and thereby forming ferrous sulfate.

The pickling process of this invention has been found to produce much more uniform pickling than has been possible with prior methods of control in conventional solutions. This uniform degree of etching or metal removal enables processors topreliminarily calculate the desired quantity of metal loss as a preliminary step in any of the conventional processes using subsequent treatments including electroplating, phosphate coatings, oxide coating, application of copper Hash coatings, nickel coatings by electroless methods or the like, and the preliminary etching method of this invention has been found to be unusually satisfactory and beneficial in processes which form a vitreous enamel coating on the pickled ferrous surface.

The process may be carried out in the plant of the manufacturer of the ferrous surface to be treated, such as a sheet metal manufacturer, or it may be carried out at the site of any of the subsequent treatments above enumerated. Moreover, the pickling step of this invention may be performed on articles or products which have been preliminarily shaped into the form of a iinal product, and for the purposes of this invention the term article as used in the appended claims is intended to encompass metal surfaces in the form of sheet, strip, or a linally formed article.

While the preferred form of this invention utilizes a phosphoric acid solution containing only ferrous and ferric ions, satisfactory results can be obtained in the presence of minor quantities of other mineral acids, such as sulfuric, hydrochloric acids, etc. The quantities of any such other acids should be limited to minor quantities of, for example, less than about 50% Vof the total.

It is also feasible to aid control and to reduce the quantity of ferrous ion which is dissolved in the pickling solution as the result of the attack of the solution on the metal surface by incorporating in the pickling solution one of the conventionally employed inhibitors such,

for example, as dibutylthiourea, di-isopropylthiourea, sodium iodide, Rodine 82 or Rosin Amine D etc. The quantity of metal dissolved in a 15 point pickling solution containing 0.01% dibutylthiourea was only approximately one-half the quantity in the absence of the inhibitor. The surf-ace preparation was nevertheless of indistinguishable character to that resulting from the use of solutions comparable in all respects except for the presence of the inhibitor. By the term point is meant the number of cc.s of N/ 10 sodium hydroxide solution required to titrate l m1. of the pickling solution to a bromphenol blue indicator end joint.

To employ a ferrous surface pickled in accordance with the method of this invention in conjunction with the subsequent formation of a nickel ash coating or a nickelcobalt alloy coating, or the like, it is necessary only to rinse the surface taken lfrom the phosphoric acid pickling solution of this invention thoroughly with Water and thereafter use any of the conventional nickel application processes which are conventional in the art. As an example, the pickled surface may be provided with a nickel coating by electroless procedures, such as those disclosed in U.S. Patent 2,532,283, of December 5, 1950, or may be coated with a cobalt plate deposited by the method of U.S. Patent 2,532,284, of December 5, 1950. These processes employ an aqueous solution of a cobalt or a nickel salt, such as cobalt -or nickel chloride, in combination with controlled quantities of hypophosphite radicalcontaining material. If the subsequent coating is to be neckel-cobalt alloy coating, this coating may be obtained by using the procedure of U.S. Patent 2,639,264, of May 19, 1953, by employing a bath of substantially the following composition: 200 grams NiSO4.6H2O, 37.5 grams COSO4.6H2O, 20 grams boric acid 3.8 grams citric acid, 3.75 grams dextrose and 25 cc. of a product made by dissolving 40 lbs. of sodium hydroxide in 81/2 gal. of water, adding -Cr03, and then mixing with 37 gallons of 50% glutamic acid, and 50 mgs. sodium bisulte (NaI-i503) made up to 2 gallons of water. In using this solution the bath is maintained at a pH of 4.9 to 5.1 and the pickled surface is contacted with the solution for 15 to 20 minutes.

Such a nickel-coated, cobalt-coated or nickel-cobalt alloy-coated surface is in satisfactory condition to be further coated with a vitreous enamel coating. It has been found advantageous in some vitreous enamel processes to preliminarily heat-treat the nickel or nickel-cobalt overlayer prior to the application of the vitreous enamel frit. The coated surfaces may be heat-treated by raising the temperature to between about ll00 F. and 1900 F., maintaining the temperature for a short period of time, for example, 1 to 10 minutes. As the temperature is increased toward the 1900 F. the time required to obtain the benets of heat-treatment of the nickel coating is reduced, and as a general guide, the time may be reduced between 1 and l0 minutes proportionately as the temperature increases from 1100 F. to 1900 F. The atmosphere is preferably slightly oxidizing, such as may be induced by raising the temperature of the surface in air -or heating the nickel coated surface in the presence i of moisture, such as steam or as a result of raising the temperature while the surface is still wet. The heattreated nickel or nickel-cobalt alloy surfaces have Abeen found to provide somewhat better adherence for subsequently applied vitreous enamel than corresponding nickel or nickel-cobalt alloy coatings which have not been heattreated.

The etched surface resulting from the pickling process of this invention is especially adapted for the subsequent application of or conversion to a phosphate-oxide coating as an underlayer for the nal coating of the surface with a vitreous enamel layer which process is illustrated in the drawing. For the purpose of forming a conventional phosphate coating or additional phosphate coating, where needed to obtain the desired phosphate-oxide coating, a conventional phosphate coating solution may be used, such as a zinc, or manganese acidic phosphate solution, an iron phosphate solution, or an iron phosphate-producing solution, such as a sodium, potassium or ammonium phosphate solution. In producing a phosphate coating which is thereafter converted into a mixed phosphateoxide coating, it is of critical importance to control the :onditions of the phosphate coating step such that the total phosphate coating formed on the surface does not exceed about 580 mgs. per sq. ft. and the P04 portion of that phosphate coating lies in the range of about 1 mg. to about 212 mgs. per sq. ft. With such a coating on the surface, the surface is then ready to be subjected to an oxidizing atmosphere raised temperature treatment to convert at least a portion of the phosphate coating into an oxide-phosphate coating. A satisfactory temperature in air is in the range of about l100 F. to about 1900o F. and preferably about l350 F.-l465 F. for a short :ime, for example, about 3 to l5 minutes. As a result 3f this heat treatment the coating should have a total weight in the range of about 0.4 gram and 42 grams/sq. ft. The phosphate-oxide coated surface is then in condition for receiving conventional vitreous enamel coatings. Suitable porcelain enamels for this purpose include the alkali-aluminum-fluoborosilicate glasses which are tired in a temperature range of l470 F. to 1560 F., and titanium-containing enamels. A suitable one layer enamel application results from using the titanium-containing enamels. Such titanium-containing enamels may be described as titanium opacied porcelain enamel comprising essentially an alkali-titanium-iiuobrosilicate glass susceptible to fusion on a metal surface at a ternperature in the range of 1450 F. to 1550 F. During the fusion operation titanium dioxide is precipitated, giving a white coating of high opacity. Such vitreous enamels may optionally contain various color-producing compounds, and it is to be understood that colored enamels work as satisfactorily as the white enamels. The enamel may be applied in any conventional manner, such as by dipping or spraying the phosphate-oxide coated article and the finished article is obtained by properly firing the vitreous enamel coating.

As above indicated the actual steps taken to maintain the process in a steady state condition are steps which control the quantity of ferrous ion in the pickling bath or the quantity or rate of ferrous ion addition to the bath as the result of continued operation. The quantity of ferrous ion in the bath is controlled by altering the rate of flow of the solution through the ion exchange bed, whereas the rate and/or quantity of ferrous ion added as the result of continued operation is controlled by altering :he rate or quantity of the solution being sprayed, the quantity of oxygen available to the spray, modifying the rate of throughput or decreasing the rate of metal attack by incorporating inhibitors in the pickling solution. [irrespective of the actual step selected to achieve control, the objective of positively exercising the step is to restore the steady state operation to accommodate the change in the variable which caused the migration from the steady state condition. Migration from steady state condition and the amount thereof is determined lby making periodic analyses of the pickling solution for acidity, ferrous and ferric ion concentration. The time interval between analyses will vary with the conditions of each installation, but typical commercial operating conditions are satisfied by two analyses per hour. However, when known changes occur it is best to analyze shortly thereafter and follow with analyses every l5 minutes until the effect of the change becomes apparent. It will be apparent that consecutive analyses will rarely be identical, but it has been found that satisfactory control is maintained by initiating appropriate steps at the point when the ratio of ferrous to ferric ion has migrated from 20% to 30% of the value of the ratio under steady state conditions. `On the other hand, if no corrective steps are taken to restore the steady-state ratio until the ratio has migrated as much as 50% to 100% from the steady state value, the cascading effect of the ferrous ion content on the ferric ion content is greatly amplified, and there is an enhanced risk of reaching a heavy sludging condition, where the migration was an increase in ferrous ion, or of reaching low efficiency in metal attack, where the migration was a decrease in ferrous ion. The simplest and thus the preferred step to employ when a migration from steady state conditions is detected is that of varying the rate of flow through the ion exchange bed, and in fact it is feasible to control any known commercial process by appropriately using this single positive step in the manner indicated. lt will be understood that reduction in spray rate and spray pressure, addition of inhibitor and increase in rate of solution flow through the ion exchange bed all tend to reduce ferrous and ferric ion content on continued operation, and that any selected combination of such steps may be used. lt is also clear that an increase in ferrous and ferric ion results from decreasing the rate of ow through the ion exchange resin bed, and/or increasing the spray rate, spray pressure and oxygen in the spray chamber. For emergency operation an additional spray chamber separate from the chamber in which the work is pickled can be used, and requires only appropriare piping and pumping apparatus for bleeding off a portion of the pickling solution, spraying the same and returning the oxidized solution to the pickling solution reservoir.

The below-given examples are intended to illustrate typically satisfactory operating conditions for carrying out the pickling process of this invention and for combining that process with subsequent coatings with other final or intermediate coatings. It is to be understood that the particular solutions and conditions employed are not to be considered as limitative of the invention but rather as illustrative only.

Example I A 1500 gallon aqueous solution was prepared containing 15% H3PO4 w./v. and placed in :a commercial spraying installation designed to spray pickle sheet metal articles sucn as refrigerator bodies, washing machine housings, etc. The spray section comprised a four sided housing, 4' x 6, open at each end, and containing banks of spray nozzles adjacent to each of the four walls and spaced on 12 centers, the entire section containing 56 operative nozzles of the type which spray in a V-shaped pattern. The nozzles were designated l/4 U 5070 VzJ, obtained from Spraying Systems Co., and produce a 50 angle spray when operated at 40 p.s.i.g. The solution was pumped through the leaders and to the nozzles under a pressure of 20-25 psi., and the drain solution recirculated. Attached to the pickling reservoir were two sets of ion exchange columns consisting of two columns in series, with each set `being ion exchange columns piped to the return reservoir and provided with a suitable controlled ow pump and suitable valves to permit switching from one pair to the other pair to permit regeneration of the spent resin as required. Each column was filled with 25 cubic feet of Dowex 50 resin, hydrogen form. With the solution temperature at an average temperature of about F., low carbon iron, free of grease and scale was sprayed at a throughput rate of approximately l-1.2 sq. ft. per minute for 72 hours and pickling solution was fed through the ion exchange bed at the rate of 6 gallons/minute. After 50 hours, the ferrous ion concentration was 0.06% and the ferric ion concentration was 0.12%. Continuing the same operation an analysis after 69 hours showed the ferric ion concentration to be 0.11% and the ferrous ion concentration to be 0.07%. The pickling rate was considered to be relatively slow and the bath was increased in H3PO4 concentration to convert the entire bath to a 25% H3PO4 value. With similar work being processed through the spray section under the same conditions the ferric ion concentration was found to be 0.21% and the ferrous ion concentration 0.06% after 75 hours. At the end of 88 hours the ferric ion concentration was 0.22 and the ferrous ion concentration was 0.035 `and at this point the flow through the ion exchange column was stopped entirely. After 89 hours 9 v the ferrous ion concentration had increased to 0.08% and the ferric ion concentration was 0.225%. After 90 hours the ferrous ion concentration was 0.08% and the ferric ion concentration was 0.25%. After 91 hours the ferrous ion concentration was 0.10 and the ferric ion concentration was 0.31. At this point the ow through the ion exchange column was again started at the rate of 6 gallons per minute.` At the end of 93 hours the ferrous ion concentration had decreased to 0.075% and the ferric ion concentration had decreased to 0.25%. The rate of ow through the io-n exchange Icolumn was then decreased to 3 gallons/minute and the rate of throughput through the spraying section was decreased from about 1.15 sq. ft./minute toi about 0.45 sq. ft./minute `and analysis at the end of 96l hours showed the ferrous ion concentration to be 0.11% and the ferric ion concentration to be 0.29% With the flow rate through the column at 3 gallons/ minute the ferrous ion concentra-tion increased to 0.11 and the ferric ion concentration increased to 0.37 at the end of 99 hours. At this point the rate of flow through the column was again increased to 6 gallons/ minute and the rate of throughput was increased to 1.92 sq. -ft./minute. Under these conditions, at the end of 101 hours the ferrous ion concentration was `0.08% and the ferric ion concentration was reduced to 0.33. At the end of 101 hours the rate of ow through the ion exchange column was again decreased to 3 gallons/minute. After 102 hours the ferrous i-on concentration had increased to 0.135 and the ferric ion concentration was 0.32%. Operating under the 3 gallons/ minute rate through the ion exchange column and at the 1.92 sq. ft./minute throughyput nate 4an analysisl after 104 hours showed the ferrous erated such that substantially 100% of the ferrous ion concentration of the solution passed through the column was removed and throughout the run the small quantity of ferric phosphate sludge which was formed in the solution and which deposited on the heating elements was subsequently redissolved by the alterations made in the ferric and ferrous ion concentrations in the pickling solution.

The lchanges in ferrous and ferric ion concentrations which occurred between 88 and 91 4hours of operation illustrate that failure toact after a substantial change in the original Iratio of ferrous to ferric ion occurs causes a continuing increase in the ferric ion concentration. At the end of 90 hours it was 1apparent that the migration from the steady state condition was sufficient to indicate that a `control step was needed to reduce the tendency for ferrous and ferric ions to increase in the solution. No action was taken relative to 'beginning the ow of solution through the ion exchange column for an additional hour and between the 90th and 91st hours the ferric ion concentration increased from 0.25% to `0.32% and at thi-s point the ion exchange column flow was again initiated. After 911/2 hours the ferric ion concentration was reduced to 0.28% and the ferrous i-on concentration reduced to 0.11% thus demonstrating the fast remedial effect of flow of the solution through the ion exchange column. At the end of two additional hours, or at the 93rd hour, both the ferrous and ferric ion concentrations had substantially reduced.

Operation between the 93rd and 99th hours demonstrated the concurrent effect of increasing the rate of throughput and decreasing the rate of flow through the ion exchange column.

Between the 99th and 102nd hours the effect of increasing the rate of flow through the ion exchange 1t) column and simultaneously increasing the throughput was demonstrated and between the 102nd and 104th hours the effect of decreasing the rate of flow through the ion exchange column with the rate of throughput at the new higher rate is illustrated.

A plurality of panels of the enamelling iron pickled in the spray section between the 93rd and 99th hours of operation, after rinsing were contacted with la sodium dihydrogen phosphate aqueous solution containing 12.5 grams/liter monosodium phosphate land having a pH of 5.2 for one minute at about 140 F. The panels were then positioned in a mutlie furnace, having its door partially open, zat a temperature of 1350 F. and maintained therein for 3f minutes. After being removed from the furnace the panels were sprayed with a vitreous enamel slip until the slip layer averaged about 30 grams/sq. ft. on the surface. The enamel slip was a mixture containing in part-s by weight:

The panels were inserted in a rnuffle furnace at a temperature of 10.80 F. and maintained therein for 4 minutes and upon withdrawal were observed to be coated with a white glossy enamel finish. The panels were tested for resistance to torsion, impact and deformation, and in each category found to be satisfactory.

Example Il Y Another series of the enamelling iron parts processed between the 93rd and 99th hours of operation were, after rinsing, immersed in an electroless nickel bath containing, in parts by Weight:

Nickel chloride (NiClgHzO) 3 Sodium hypophosphite (NaHzPOZHZO) 1 Ammonium chloride 5 Sodium citrate 10 Water 81 pH: 8 to 9.

and maintained therein at a temper-ature of about C., for l0-15 minutes and withdrawn. A plurality of the panels so treated were positioned in a muffle furnace, having its door partially open, at a temperature of 1350 F. and maintained therein for 3 minutes. A plurality of these nickel coated panels, both red and unred were then coated with the vitreous enamel slip above referred to and in the same manner and upon testing for torsion, deformation and impact were found to be satisfactorily resistant to torsion forces, to the impact of the ball end of a hammer which had made a depression of about 0.15 in depth, and to the P.E.I. bump test.

Example III A further series of panels processed between the 93rd and 99th hours of operation were, after rinsing coated with a vitreous enamel slip directly overlying the rinsed pickled surface. The vitreous enamel slip was applied -by spraying to form a slip layer averaging about 30 grams/sq. ft. on the surface. The vitreous enamel slip used for this purpose contained in parts by weight:

rIhese panels were then fired for 6i minutes at 1300 F.

l 1 and upon withdrawal from the furnace were subjected to torsion, deformation and impact tests of the same type as above described in Examples I and Il. Good resistance to torsion, impact and deformation, by the P.E.I. bump test, was obtained.

Example IV A still further series of panels of the enamelling iron pickled in the spray section between the 93rd and 99th hours of operation, after rinsing, were immersed for 5 to 8 minutes in an aqueous nickel sulfate solution containing about 2 ozs. of NiSO4.6H2O and Mi ounce boric acid per gallon of Water. The nickel sulfate solution was maintained at a temperature of about 160 F.-180 F. and had a pH of about 4.0. This treatment produced a nickel coating having a weight of about 0.03 to about 0.1 gram/sq. ft. After rinsing, these panels were coated with the vitreous enamel slip described in Example I under the conditions of application and firing there used. The resulting panels were uniformly coated with a white glossy enamel finish which had satisfactory torsion impact and deformation resistance. A number of similar panels were nickel-flash coated by spraying a nickel sulfate solution on the pickled surface, the nickel solution containing 2 ounces of NiSO4.6H2O per gallon of water, a pH of 3 to 3.5 and sprayed at a temperature of 170 F.- 180 F. These panels when coated with a comparable vitreous enamel coating were found to have substantially similar properties to those obtained on the panels coated with nickel from a nickel-sulfate immersion bath.

This application is a continuation-in-part of application Serial 721,628, filed March 17, 1958, now abandoned.

What is claimed is:

1. A method of picking metallic surfaces which cornprises the steps of spraying said surfaces with an aqueous acidic phosphoric acid solution, said solution containing the ferrous ion and between about 13% and about 85 of the saturation concentration of the ferric ion, the ratio of ferrous ion to ferric ion being in the range of about 11:1 to -about 1:7, and as said metallic surfaces are continuously processed through said solution controlling the concentration of ferrous ion so as to continuously maintain the ferric ion concentration below its saturation value.

2. A method of picking ferrous surfaces which comprises the steps of spraying on said surfaces an aqueous acidic solution deriving at least 50% of its acidity from phosphoric acid, said solution containing the ferrous ion and at least about 13% of the saturation concentration of the ferric ion, the ratio of ferrous ion to ferric ion being in the range of about 11:1 to about 1:7, and as ferrous surfaces are processed through said spray, circulating a portion of the said solution through an ion exchange resin bed capable of removing the ferrous ion therefrom and returning the ferrous ion free solution to said spraying solution, and controlling the rate of llow of said solution through said ion exchange bed so as to continuously maintain the ferric ion concentration in said spray solution in the range. of about 13% to about 85% of the ferric ion saturation value of said solution.

3. A method of pickling ferrous surfaces which cornprises the steps of spraying on said surfaces an aqueous acidic solution deriving at least 50% of its acidity from phosphoric acid, said solution containing the ferrous ion and at least about 13% of the saturation concentration of the ferric ion, the ratio of ferrous ion to ferric ion being in the range of about 11:1 to about 1:7, and as ferrous surfaces are processed through said spray, controlling the oxidation conditions of the ferrous ion to the ferric ion so as to bring the ferric ion concentration within the range of about 501%-85% of its saturation value to thus establish a steady-state condition, periodically analyzing the said solution for ferrous and ferric ion concentration and upon determination that the ferrous and ferric ion concentrations representing steady-state conditions have migrated therefrom by %-30% of the steady-state value, altering the ferrous ion concentration 'so as to restore said steadystate concentrations as ferrous surfaces continue to be processed through said spray.

4. A method of pickling metallic surfaces in accordance with claim 3 wherein said solution contains between about 15% and 45% H3PO4 and the ratio of ferrous to ferric ion in the solution is maintained within the range of about 1:4 to about 4:1.

5. A method in accordance with claim 3 wherein the step of altering the ferrous ion content involves the cycling of a portion of said spray solution through an ion exchange resin bed capable of removing the ferrous ion therefrom and returning the ferrous-free solution to said spray solution.

6. A method of preparing a metallic surface to receive a vitreous enamel coating which comprises the steps of spraying said surface with an aqueous acidic phosphoric acid solution, said solution containing the ferrous ion and between about 13% and about 85 of the saturation concentration of the ferric ion, the ratio of ferrous ion to ferric ion being in the range of about 11:1 to about 1:7, and as said metallic surfaces are continuously processed through said solution controlling the concentration of ferrous ion so as to continuously maintain the ferric ion concentration below its saturation value, and thereafter forming a nickel flash coating on said pickled surface.

7. A method of forming a vitreous enamel coating on a metallic surface which comprises the steps of spraying said surface with an aqueous acidic phosphoric acid solution, said solution containing the ferrous ion and between about 13% and about 85% of the saturation concentration of the ferric ion, the ratio of ferrous ion to ferric ion being in the range of about 11:1 to about 1:7, and as said metallic surfaces are continuously processed through said solution controlling the concentration of ferrous ion so as to continuously maintain the yferric ion concentration below its saturation value, applying a nickel ash coating to said pickled surface and thereafter covering said nickel coated surface with at least one layer of vitreous enamel and tiring the same.

8. A method of forming a vitreous enamel coating on a metallic ferrous surface which comprises the steps of spraying said surface with an aqueous acidic phosphoric acid solution, said solution containing the ferrous ion and between about 13% and about 85 of the saturation concentration of the ferric ion, the ratio of ferrous ion to yferric ion being in the range of about 11:1 to about 1:7, and as said metallic surfaces are continuously processed through said solution controlling the concentration of ferrous ion so as to continuously maintain the ferric ion concentration below its saturation value, contacting the pickled surface with an aqueous acidic phosphate solution to form on the surface a phosphate coating not exceeding 580 mg./sq. ft. in weight, the P04 portion of said phosphate coating being in the range of about l mg. to about 212 mg/sq. ft., heating said phosphate coating in air at a temperature in the range of about 1100 F. to about 1900" F. for a short time so as to form a coating having a total weight in the range of about 0.4 gram/sq. ft. to about 42 grams/sq. ft., and thereafter coating said surface with a vitreous enamel and tiring the same.

9. A method of forming a vitreous enamel coating on a ferrous surface which comprises the steps of spraying said surface with an aqueous acidic phosphoric acid solution, said solution containing the ferrous ion and between about 13% and about 85% of the saturation concentration of the ferie ion, the ratio of ferrous ion to ferric ion being in the range of about 11:1 to about 1:7, and as ferrous surfaces are continuouslf,I processed through said solution controlling the concentration of ferrous ion so as to continuously maintain the ferric ion concentration below its saturation value, forming a nickel ash coating on said pickled surface, heating said nickel coating in air at a temperature in the range of about ()D F. to about 13 1900 F. vfor a short time and thereafter covering said surface with a vitreous enamel frit and rin-g the same.

References Cited in the file of this patent UNITED STATES PATENTS 2,127,388 Canfield et a1. Aug. 16, 1938 2,532,283 Brenner Dec. 5, 1950 2,559,445 Lotz July 3, 1951 

1. A METHOD OF PICKING METALLIC SURFACES WHICH COMPRISES THE STEPS OF SPRAYING SAID SURFACES WITH AN AQUEOUS ACIDIC PHOSPHORIC ACID SOLUTION, SAID SOLUTION CONTAINING THE FERROUS ION AND BETWEEN ABOUT 13% AND ABOUT 85% OF THE SATURATION CONCENTRATION OF THE FERRIC ION, THE RATIO OF FERROUS ION TO FERRIC ION BEING IN THE RANGE OF ABOUT 11:1 TO ABOUT 1:7, AND AS SAID METALLIC SURFACES ARE CONTINUOUSLY PROCESSED THROUGH SAID SOLUTION CONTROLLING THE 